1. Field of the Invention
The present invention relates to a liquid crystal display device, particularly to a liquid crystal display device in which a protrusion to serve as a spacer is formed on a substrate.
2. Description of the Related Art
In a general-type liquid crystal display device, liquid crystal is sandwiched by two glass substrates on which electrodes are formed, and the two substrates are fixed together with an adhesive agent. Moreover, in an active matrix liquid crystal display device, which is capable of color display, multiple signal lines and multiple scanning lines are disposed on one of the substrates, which is an array substrate, in a manner that each signal line crosses the scanning lines. On the intersections of these lines, pixel electrodes and thin-film transistors (TFTs) each including a semiconductor layer made of amorphous silicon (a-Si) are disposed, respectively. On the other substrate, which is an opposite substrate, an opposite electrode and color filters are formed. In addition, on both outer sides of the two substrates, polarizing plates, which function as an optical shutter, are disposed.
In addition, plastic beads with the same particle size are interspersed between the two substrates such that a distance (cell gap) between the substrates can be kept constant. Recently, a method is also being used in which protrusions (spacers) are formed on the substrates such that the cell gap can be kept constant. Since the spacers can be disposed selectively in any positions, and have high height accuracy, they are effective at uniforming the cell gap. More recently, as a measure against an external load temporarily applied on a liquid crystal display device, disclosed is a technique of providing, as auxiliary spacers, one of the substrates with columnar protrusions which are not in contact with the opposite one of the substrates under normal conditions (refer to Japanese Patent No. 3680730, for example).
However, when an external impact is applied on a conventional liquid crystal display device in a low-temperature environment, air bubbles may possibly be generated in a liquid crystal layer therein. The linear expansion coefficient of liquid crystal is larger than those of spacers and substrates. Accordingly, at a low temperature, a contraction rate of the liquid crystal is significantly different from those of the spacers and the substrates. Thus, when an external impact is applied on a conventional liquid crystal display device at a negative pressure in a low-temperature environment, the liquid crystal cannot contract along with the contraction of the spacers and the substrates in the liquid crystal display device. As a result, air bubbles are generated in the liquid crystal, which causes a problem that display quality of the conventional liquid crystal display device is seriously deteriorated.
To address this problem, methods for suppressing generation of such low-temperature bubbles have been proposed. In one of these methods, disposition density of spacers is reduced so that the substrates can be more flexible. However, with too reduced disposition density, spacers will not exert their principle function, and thus liquid crystal cells are likely to be narrowed locally or generally. Accordingly, the method has a problem of causing cell gap defects and defects in optical characteristics in a liquid crystal display device, and thus deteriorating display quality of the liquid crystal display device. Meanwhile, as another effective method for suppressing generation of low-temperature bubbles, there is a method of increasing a liquid crystal amount to be injected into a liquid crystal cell in order to reduce a negative pressure generated when the liquid crystal contracts in a low-temperature environment, in other words, a method of injecting a large amount of liquid crystal into a liquid crystal cell by applying a pressure thereon. However, too large amount of liquid crystal will make a cell gap nonuniform in a display. Accordingly, the method has a problem of causing display defects of a liquid crystal display device, and thus deteriorating display quality thereof.